FIGS. 6(a) to 6(e) are cross-sectional views illustrating process steps of a prior art method for producing a semiconductor laser. In figures, reference numeral 1 designates an n type GaAs substrate. An n type AlInGaP cladding layer 2 is disposed on the substrate 1 and an InGaP active layer 3 is disposed thereon. A p type AlInGaP cladding layer 4 having a mesa ridge is disposed on the cladding layer 4 at the active layer 3. An n type GaAs layer 5 is disposed on both sides of the mesa ridge. A p type GaAs layer 6 is disposed on the n type GaAs layer 5 and the p type AlInGaP cladding layer 4. An n side electrode 7 and a p side electrode 8 are disposed on the rear surface of the substrate 1 and the surface of the p type GaAs layer 6, respectively.
First, as shown in FIG. 6(a), an n type AlInGaP cladding layer 2, an InGaP active layer 3, and a p type AlInGaP cladding layer 4 are successively grown on the GaAs substrate 1 (first crystal growth). Generally, silicon is added to the n type AlInGaP cladding layer 2 as an n type dopant and zinc is added to the p type AlInGaP cladding layer 4 as a p type dopant. Then, an etching mask 10 is patterned on the p type AlInGaP cladding layer 4 and the p type AlInGaP cladding layer 4 is partially etched away using the etching mask 10 leaving a stripe-shaped mesa, as shown in FIG. 6(b). Thereafter, as shown in FIG. 6(c), an n type GaAs layer 5 is formed on both sides of the stripe-shaped mesa using the mask 10 as a mask for the selective growth (second crystal growth). After removing the mask 10, a p type GaAs layer 6 is formed on the mesa and the n type GaAs layer 5 as shown in FIG. 6(d) (third crystal growth). Finally, an n side electrode 7 and a p side electrode 8 are formed on the rear surface of the n type GaAs substrate 1 and the surface of the p type GaAs layer 6, respectively, completing a laser structure shown in FIG. 6(e).
A description is given of the operation.
When a forward bias voltage is applied across the n side electrode 7 and the p side electrode 8, current flows between the electrodes 7 and 8. This current produces electrons in the n type cladding layer 2 and holes in the p type cladding layer 4, and these electrons and holes are injected into the active layer 3 and recombine with each other, thereby generating light whose wavelength corresponds to the energy band gap of the active layer 3, i.e., approximately 670 nm.
When the current flowing in the laser increases and reaches a threshold value, laser oscillation begins and laser light having a wavelength corresponding to the energy band gap of the active layer 3 is produced. When the active layer 3 is formed of InGaP, red laser light having a wavelength of approximately 670 nm is produced. After the current exceeds the threshold value, the output laser light increases in proportion to the increase in the current.
In the above-described prior art semiconductor laser, selenium or silicon is employed as an n type dopant for the n type AlInGaP cladding layer 2 and zinc is employed as a p type dopant for the p type AlInGaP cladding layer 4.
However, the prior art semiconductor laser has the following drawbacks:
Zinc atoms and selenium atoms are likely to move in the layers at a high temperature. Therefore, as shown in FIGS. 6(c) and 6(d), zinc atoms in the p type cladding layer 4 are likely to diffuse into the active layer 3 at the high temperature of the second and third crystal growth processes. Similarly, when selenium is used as an n type dopant, selenium atoms in the n type cladding layer 2 are likely to diffuse into the active layer 3 at the high temperature of the second and third crystal growth processes. As shown in FIG. 7, zinc atoms or selenium atoms diffused into the active layer 3 form a deep impurity level 14. Usually, electrons in the conduction band directly recombine with holes in the valence band in the active layer 3 as shown in FIG. 7 and light of wavelength 1 corresponding to the energy band gap of the active layer 3 is produced. However, since the deep impurity level 14 is present in the active layer 3, when electrons in the n type cladding layer 2 and holes in the p type cladding layer 4 are injected into the active layer 3 by a forward direction current, the electrons 12 in the conduction band recombine with the holes 13 in the valence band not directly but via the above-described deep impurity level 14. Such a recombination of electrons and holes generates light of wavelength .lambda..sub.2 which does not contribute to the laser oscillation. In order to oscillate, it is necessary to sufficiently generate the light of wavelength .lambda..sub.1 which corresponds to the energy band gap of the active layer 3. For that purpose, the electrons in the conductive band and the holes in the valence band recombine directly with each other only after saturating the recombination process via the deep impurity level 14. In this case, however, the threshold current unfavorably rises.